Biased bang-bang phase detector for clock and data recovery

ABSTRACT

An apparatus includes a plurality of phase detector circuits and a summing circuit. Each of the plurality of phase detector circuits may be configured to generate a phase up signal and a phase down signal in response to a respective pair of data samples and intervening transition sample. The summing circuit may be configured to generate an adjustment signal in response to the phase up and phase down signals of the plurality of phase detector circuits. A sum of the phase up signals and a sum of the phase down signals are weighted to provide a bias to a phase adjustment.

This application relates to U.S. Provisional Application No. 61/790,046,filed Mar. 15, 2013, which is hereby incorporated by reference in itsentirety.

FIELD OF THE INVENTION

The invention relates to data recovery generally and, more particularly,to a method and/or apparatus for implementing a biased bang-bang phasedetector for clock and data recovery.

BACKGROUND

Clock and Data Recovery (CDR) circuits form a critical part of receiversin serializer-deserializer (SerDes) communication channels. The CDRcircuits are used to track the phase of a sampling clock based on somecriterion like minimizing mean squared-error (MSE). It is important thatthe CDR circuits operate well enough to achieve a very low targetbit-error rate (BER) on the order of 1e-12 or 1e-15. The CDR circuits incommon use can be broadly classified into two categories, baud-rate CDRand bang-bang CDR. There are advantages and disadvantages associatedwith each category.

In bang-bang (or Alexander) type CDR, a received signal is sampled twiceevery symbol period, also called a unit interval (UI). Ideally onesample is at a crossing boundary and another sample is at a center of areceiver data eye. Two consecutive data samples (V[K−1] and V[K]) andone transition (or crossing) sample (V[K−½]) between the two datasamples are used to decide whether a current sampling phase is laggingor leading an ideal sampling point. The sampling phase is then correctedaccordingly. In a CDR circuit using a conventional bang-bang phasedetector (BBPD), the phase of a transition sampling clock settles at themedian of the jitter distribution after convergence.

SUMMARY

The invention concerns an apparatus includes a plurality of phasedetector circuits and a summing circuit. Each of the plurality of phasedetector circuits may be configured to generate a phase up signal and aphase down signal in response to a respective pair of data samples andintervening transition sample. The summing circuit may be configured togenerate an adjustment signal in response to the phase up and phase downsignals of the plurality of phase detector circuits. A sum of the phaseup signals and a sum of the phase down signals are weighted to provide abias to a phase adjustment.

BRIEF DESCRIPTION OF THE FIGURES

Embodiments of the invention will be apparent from the followingdetailed description and the appended claims and drawings in which:

FIG. 1 is a diagram illustrating a communication channel implementing anembodiment of the invention;

FIG. 2 is a diagram illustrating an example of a bang-bang phasedetector (BBPD) implemented in accordance with an embodiment of theinvention;

FIG. 3 is a table illustrating an input/output relationship of the BBPDof FIG. 2;

FIG. 4 is a diagram illustrating another example of a bang-bang phasedetector (BBPD) implemented in accordance with an embodiment of theinvention and configured to lock a phase of a transition sampling clockto a right inner corner of a receiver data eye;

FIG. 5 is a diagram illustrating the BBPD of FIG. 4 configured to lockthe phase of the transition sampling clock to a left inner corner of adata eye;

FIG. 6 is a diagram illustrating a gain versus phase error relationshipfor various threshold settings in a bang-bang phase detector implementedin accordance with an embodiment of the invention;

FIG. 7 is a diagram illustrating a transition sampling phase settling ata left inner corner of the receiver data eye;

FIG. 8 is a diagram illustrating a transition sampling phase settling ata right inner corner of the receiver data eye; and

FIG. 9 is a diagram illustrating phase convergence behavior of the phaseof the transition sampling clock for various threshold values.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the invention include method and/or apparatus forimplementing a biased bang-bang phase detector (BBPD) in clock and datarecovery (CDR) applications. The biased bang-bang phase detector inaccordance with embodiments of the invention generally allowsconvergence to be biased to a right or left inner corner of a receiverdata eye depending on a pair of control parameters. In variousembodiments, by setting the control parameters to particular values, thephase of a transition sample clock is locked at the inner left corner orinner right corner of the receiver data eye at any desired bit errorrate (BER). In various embodiments, a ratio between the two controlparameters is set according to the desired BER.

Referring to FIG. 1, a block diagram illustrating aserializer/deserializer (SerDes) communication channel 90 including abiased bang-bang phase detector (BBPD) in accordance with an embodimentof the invention is shown. The SerDes channel 90 has a channelimpairment that is due, for example, to a physical transmission medium,such as a backplane or drive head in a magnetic recording system. Insome embodiments, the channel 90 is generally configured to operate atsome predetermined baud-rate. In the exemplary implementation shown inFIG. 1, a signal carrying data (e.g., DATA) is optionally equalized orfiltered through a transmit finite impulse response (FIR) filter (TXFIR)91 prior to being transmitted through a channel 92. After passingthrough the channel 92, a received signal may optionally be filtered orequalized by a receive equalizer (RXEQ) 93. The RXEQ 93 may beimplemented as, for example, a continuous time filter. An output signalof the RXEQ 93 is sampled at the baud rate by a switch 94 using a datasampling clock signal (e.g., D_CLK) and a switch 95 using a transition(or crossing) sampling clock signal (e.g., T_CLK).

In some embodiments, a data detector 96 (or a slicer) digitizes thesample from the switch 94 and compares the digitized sample to anexemplary threshold (e.g., zero), using the clock D_CLK. A crossingdetector 97 (or a slicer) digitizes the sample from the switch 95 andcompares the digitized sample to an exemplary threshold (e.g., zero),using the clock T_CLK. The sampling clocks (or phases) D_CLK and T_CLKare generated by a clock and data recovery (CDR) circuit 100 based upondetected data (e.g., DETECTED DATA) and transition data (e.g.,TRANSITION DATA) generated by the data detector 96 and transitiondetector 97, respectively. In some embodiments, the circuit 100comprises a block (or circuit) 102 and a block (or circuit) 104. Invarious embodiments, the block 102 implements a biased bang-bang phasedetector (BBPD) circuit in accordance with embodiments of the inventionand the block 104 implements a clock and data recovery loop filter andclock generation circuit.

The phase of the received signal waveform is typically unknown and theremay be a frequency offset between the frequency at which the originaldata was transmitted and the nominal receiver sampling clock frequency.In some embodiments, the function of the CDR circuit 100 is to properlysample the analog waveform of the received signal such that when thesampled waveform is passed through the data detector 96, the data isrecovered properly despite the fact that the phase and frequency of thetransmitted signal is not known. The CDR circuit 100 is often anadaptive feedback circuit and the feedback loop needs to adjust thephase and frequency of the nominal clock to produce a modified recoveredclock that can sample the received signal waveform to allow proper datadetection.

In some embodiments, the data detector 96 is implemented as a slicer(e.g., a decision device based on an amplitude threshold) or a morecomplicated detector such as a sequence detector. For high speedapplications, the data detector 96 is often implemented as a slicer thatis clocked by the data sampling clock D_CLK from the CDR circuit 100. Insome embodiments, in addition to sampling the data signal, the datadetector 96 essentially quantizes the signal to a binary “1” or “0”based on the sampled analog value and a slicer threshold (e.g., S). Ifthe input to the data detector 96 at time K is W(K), then the output(e.g., V(K)) of the data detector 96 is given by the following equation:

$\begin{matrix}{{V(K)} = {{1\mspace{14mu}{if}\mspace{14mu}{W(K)}} > S}} \\{= {0\mspace{14mu}{{otherwise}.}}}\end{matrix}$

In various embodiments, the CDR circuit 100 may comprise severalcomponents, such as a phase detector (PD), a loop filter, and a clockgeneration circuit. As shown in FIG. 1, the exemplary CDR circuit 100comprises the phase detector 102 embodied as a biased bang-bang phasedetector (BBPD) circuit and the loop filter 104 embodied as a CDR loopfilter and clock generator (a digital loop filter). The biased BBPD 102processes several quantities to compute an estimate of timingadjustments needed to sample the received signal. The timing adjustmentsfor the data sampling clock D_CLK are provided by a data sampling phaseadjustment signal (e.g., D_ADJ). In some embodiments, the timingadjustments of the data sampling clock are performed using conventionaltechniques. The timing adjustments for the transition sampling clockT_CLK are provided by a signal (e.g., T_ADJ). The transition samplingclock T_CLK is normally offset from data sampling clock D_CLK by half ofa baud period. In various embodiments, the timing adjustments of thetransition sampling clock are performed using the techniques describedbelow.

In some embodiments, the timing adjustments provided by the BBPD 102 arefiltered by the CDR loop 104 before adjusting the phase of the samplingclocks D_CLK and T_CLK. The clock D_CLK and the clock T_CLK aregenerally coupled (e.g., D_ADJ is equal to T_ADJ). For the BBPD 102,there needs to be at least two sampling clocks: a data sampling clock,which samples the detected data, and a transition sampling clock that isoffset from the data sampling clock by more or less than half a baudperiod, which samples the transition data. The transition sample data isdenoted as V(K−½) to indicate the transition data is sampled relative toV(K) by a phase offset of around one-half period. In addition, the BBPD102 makes use of a one baud period delayed version of the detected data(e.g., V(K−1)) (not shown explicitly in FIG. 1). In some embodiments,the delayed data, V(K−1), can be created internally by the BBPD 102 fromV(K). In various embodiments, the input/output relationship of the BBPD102 is characterized by the look up table shown in FIG. 3. Since theBBPD 102 employs more than one sample per baud period, the BBPD 102 isclassified as an oversampled phase detector.

Referring to FIG. 2, a block diagram is shown illustrating an exampleimplementation of the circuit 100 of FIG. 1 in accordance with anembodiment of the invention. In some embodiments, the circuit 102includes a number of blocks (or circuits) 110 a-110 n and a block (orcircuit) 112. Each of the circuits 110 a-110 n may be implemented as anoversampled (bang-bang) phase detector (PD). The circuit 110 a maypresent a pair of signals (e.g., UP1, DN1). Similarly, the circuit 110 bmay present a pair of signals (e.g. UP2, DN2). Each of the circuits 110c-110 n may present a respective pair of signals (e.g., UP3, DN3; . . .; UPn, DNn) accordingly. The circuit 102 may present the signal T_ADJ inthe response to a plurality of phase adjust signals (e.g., UP1/DN1, . .. , UPn/DNn) based upon the number of phase detector circuits 110 a-110n and a pair of control parameters (e.g., T_(UP) and T_(DN)).

The circuit 112 may combine each of the outputs of the circuits 110a-110 n to generate the signal T_ADJ. By combining the outputs of thecircuits 110 a-110 n, the circuit 112 integrates the phase decisionsmade by the circuits 110 a-110 n across a number, n, of symbol periods.In some embodiments, the input/output relationship of the block 112 maybe expressed using the following equation:

${T\_ ADJ} = {{T_{UP}\left( {\sum\limits_{i = 1}^{n}{UP}_{i}} \right)} - {T_{DN}\left( {\sum\limits_{i = 1}^{n}{DN}_{i}} \right)}}$In various embodiments, the signal T_ADJ is used to control the phase ofthe transition sampling clock directly. In some embodiments, the signalT_ADJ is used to control the phase of the transition sampling clock viathe CDR loop 104.

Referring to FIG. 3, a diagram of a lookup table 200 illustrating aninput/output relationship of the phase detector 102 of FIG. 2. The valueof ρ is based on a target bit error rate (BER). The target BER is set bythe ratio of the control parameter T_(UP) to the control parameterT_(DN) (e.g., T_(UP)/T_(DN)). When the control parameter T_(UP) isgreater than the control parameter T_(DN), the phase of the transitionsampling clock is biased towards the right inner corner of the receivereye. By setting the control parameter T_(UP) equal to the controlparameter T_(DN), the circuit 112 can be made to operate like aclassical bang-bang phase detector.

Referring to FIGS. 4 and 5, diagrams of a circuit 102′ are shownillustrating another example implementation of the biased bang-bangphase detector (BBPD) 102 of FIG. 1 in accordance with an embodiment ofthe invention. Depending upon the setting of a positive threshold valueand a negative threshold value of the BBPD 102′, the phase of thetransition sampling clock can be biased towards the right inner cornerof the receiver eye (FIG. 4) or the left inner corner of the receivereye (FIG. 5).

In some embodiments, the biased bang-bang phase detector 102′ comprisesan oversampled phase detector (PD) 120, an up decision accumulator block(ACC_UP) 122, a down decision accumulator block (ACC_DN) 124, a compareblock 126, and a compare block 128. The accumulator blocks 122 and 124integrate respective phase adjustment decisions (e.g., UP or DN) fromthe phase detector 120 over a number (e.g., 8, 16, etc.) of symbolperiods. The compare block 126 determines whether the accumulated valueof up decisions exceeds (e.g., is greater than) a predetermined positivethreshold value (e.g., T_POS). The compare block 128 determines whetherthe accumulated value of down decisions exceeds (e.g., is less than) apredetermined negative threshold value (e.g., T_NEG). An output of theblock 126 and an output of the block 128 are used to adjust the phase ofthe transition sampling clock. For example, in some embodiments, theoutput of the block 126 is used to increment a phase interpolator code(e.g., PI) and the output of the block 128 is used to decrement thephase interpolator code PI. In various embodiments, the phase of thetransition sampling clock may be selected in response to the phaseinterpolator code. For example, in some embodiments, a voltagecontrolled oscillator (VCO) and a phase interpolator (PI) or a phaseselection circuit (PSC) may be implemented as part of a CDR loop filterand clock generating module (e.g., the block 104 in FIG. 1). The VCO maybe configured to generate multiphase clocks that may be used by thephase interpolator or the phase selection circuit to produce thetransition sampling clock according to the phase interpolator code.

In various embodiments, the bias of the BBPD 102′ is determined by twocontrol parameters THIGH and TLOW. For example, THIGH is set greaterthan TLOW for biased bang-bang phase detector operation and THIGH is setequal to TLOW for classical bang-bang phase detector operation. Theratio THIGH/TLOW is generally set based upon a target bit error rate(BER). In some embodiments, THIGH has a value of 256 and a lower value(e.g., 1, 16, 64, etc.) is selected for TLOW based upon the target BER.

In various embodiments and/or operation scenarios, when the magnitude ofthe predetermined negative threshold T_NEG is set to THIGH (e.g.,T_NEG=−THIGH) and the magnitude of the predetermined positive thresholdT_POS is set to TLOW (e.g., T_POS=TLOW), the BBPD 102′ is biased towardsthe right inner corner (or up). When the magnitude of the predeterminedpositive threshold is set to THIGH (e.g., T_POS=THIGH) and the magnitudeof the predetermined negative threshold is set to TLOW (e.g.,T_NEG=−TLOW), the BBPD 102′ is biased towards the left inner corner (ordown). Although the examples presented illustrate setting a bias to locka phase transition sampling clock to an inner corner of a receiver eye,the bias may bias may also be set arbitrarily to meet a design criterionof a particular application.

Referring to FIG. 6, a diagram is shown illustrating an exemplarylinearized gain versus phase error relationship for various thresholdsettings in a bang-bang phase detector implementing a biasing scheme inaccordance with an embodiment of the invention.

Referring to FIG. 7, a diagram is shown illustrating the phase of thetransition sampling clock settling at a left inner corner of thereceiver data eye. The time interval between V(K−1) and V(K−½) is equalto τ. The time interval between V(K−½) and V(K) is equal to T−τ.

Referring to FIG. 8, a diagram is shown illustrating the phase of thetransition sampling clock settling at a right inner corner of thereceiver data eye. Similar to FIG. 7, in FIG. 8 the time intervalbetween V(K−1) and the V(K−½) is equal to τ. The time interval betweenV(K−½) and V(K) is equal to T−τ.

Referring to FIG. 9, a diagram of a graph 800 is shown illustratingphase convergence behavior of the phase of the transition sampling clockfor various threshold values. A curve 802 illustrates the phaseconvergence behavior of a biased bang-bang phase detector in accordancewith an embodiment of the invention when both control parameters THIGHand TLOW are set to 256. A curve 804 illustrates the phase convergencebehavior of a biased bang-bang phase detector in accordance with anembodiment of the invention when the control parameter THIGH is set to256 and the control parameter TLOW is set to 64. A curve 806 illustratesthe phase convergence behavior of a biased bang-bang phase detector inaccordance with an embodiment of the invention when the controlparameter THIGH is set to 256 and the control parameter TLOW is set to16. A curve 808 illustrates the phase convergence behavior of a biasedbang-bang phase detector in accordance with an embodiment of theinvention when the control parameter THIGH is set to 256 and the controlparameter TLOW is set to 1.

A clock and data recovery (CDR) circuit in a receiver of a communicationsystem such as a SerDes (serializer-deserializer) aids in acquisitionand tracking of the optimal sampling phase needed for proper operationof the receiver and meeting the bit error rate (BER) performancetargets. In various embodiments, the CDR circuit is driven by a phasedetector that provides the gradient for the direction in which the phaseneeds to be adjusted. The direction of any necessary phase update ismade by considering a current data sample (e.g., V(K)), a previous datasample (e.g., V(K−1)), and a transition (e.g., V(K−½)) sample.

For a classical bang-bang phase detector (BBPD), the UP (move right) andDN (move left) are equally weighted. Hence, after convergence

Prob(V(K−½)=V(K−1))=Prob(V(K−½)=V(K)=0.5. In order to lock thetransition sampling phase at the inner left corner at BER=ρ, the samplesV(K−½)=V(K) need to match with probability 1−ρ. Hence, the sample V(K−½)is moved to the right with weight 1−ρ and moved to the left with weightρ.

In various embodiments, the weights with which the CDR phase moves tothe left or the right are determined by a pair of control parametersTHIGH and TLOW. The ratio THIGH/TLOW represents the target BER forlocking the transition sample latch to a particular inner eye corner.When THIGH equals TLOW, the ‘conventional’ BBPD relationship isobtained; moving in or out of the eye is equally probable. A higherratio THIGH/TLOW means that the eye corners are found more accurately(lower ρ), but the convergence time for the phase is longer.

The terms “may” and “generally” when used herein in conjunction with“is(are)” and verbs are meant to communicate the intention that thedescription is exemplary and believed to be broad enough to encompassboth the specific examples presented in the disclosure as well asalternative examples that could be derived based on the disclosure. Theterms “may” and “generally” as used herein should not be construed tonecessarily imply the desirability or possibility of omitting acorresponding element.

While the invention has been particularly shown and described withreference to embodiments thereof, it will be understood by those skilledin the art that various changes in form and details may be made withoutdeparting from the scope of the invention.

The invention claimed is:
 1. An apparatus comprising: a plurality ofphase detector circuits each configured to generate a phase up signaland a phase down signal in response to a respective pair of data samplesand intervening transition samples; and a summing circuit configured togenerate an adjustment signal in response to said phase up and phasedown signals of said plurality of phase detector circuits, wherein a sumof said phase up signals and a sum of said phase down signals areweighted to provide a bias to a phase adjustment, wherein saidadjustment signal is based on a difference between said sum of saidphase up signals weighted by a first control parameter and said sum ofsaid phase down signals weighted by a second control parameter.
 2. Theapparatus according to claim 1, wherein said bias is set based on atarget bit error rate of an inner eye.
 3. The apparatus according toclaim 1, wherein said phase up signals are weighted by said firstcontrol parameter, said phase down signals are weighted by said secondcontrol parameter, and a ratio of said first control parameter and saidsecond control parameter is set based on a target bit error rate of aninner eye.
 4. The apparatus according to claim 1, further comprising: aloop filter and clock generation circuit configured to adjust a phase ofa transition sampling clock according to said adjustment signal.
 5. Theapparatus according to claim 1, further comprising: at least one datadetector configured to generate said data samples in response to areceived signal and a data sampling clock; and at least one transitiondetector configured to generate said intervening transition samples inresponse to said received signal and a transition sampling clock,wherein a phase of said transition sampling clock is adjusted accordingto said adjustment signal.
 6. The apparatus according to claim 1,further comprising: at least one data detector configured to generatesaid data samples in response to a received signal and a data samplingclock; at least one transition detector configured to generate saidintervening transition samples in response to said received signal and atransition sampling clock; and a loop filter and clock generationcircuit configured to adjust a phase of said transition sampling clockaccording to said adjustment signal.
 7. An apparatus comprising: a firstsampling circuit configured to generate detected data in response to adata sampling clock; a second sampling circuit configured to generatetransition data in response to a transition sampling clock; a phasedetector circuit configured to generate a first phase adjustment signaland a second phase adjustment signal in response to said detected data,said transition data, a first control parameter and a second controlparameter, wherein (A) a point at which a phase of said transitionsampling clock settles after convergence is biased from a median valueof a receiver eye jitter distribution based on a target bit error rateof an inner eye, (B) said first control parameter sets a threshold limitfor increasing said phase of said transition sampling clock, and (C)said second control parameter sets a threshold limit for decreasing saidphase of said transition sampling clock.
 8. The apparatus according toclaim 7, wherein an amount of bias is selected based on said target biterror rate (BER).
 9. The apparatus according to claim 7, wherein saidfirst and said second control parameters are programmable.
 10. Theapparatus according to claim 7, wherein said phase detector circuitcomprises: an oversampled phase detector configured to generate a firstcontrol signal for increasing said phase of said transition samplingclock and a second control signal for decreasing said phase of saidtransition sampling clock; a first accumulator configured to generate afirst filtered control signal in response to said first control signal;a second accumulator configured to generate a second filtered controlsignal in response to said second control signal; a first comparatorconfigured to generate said first phase adjustment signal in response tosaid first filtered control signal and said first control parameter; asecond comparator configured to generate said second phase adjustmentsignal in response to said second filtered control signal and saidsecond control parameter.
 11. An apparatus comprising: a plurality ofphase detector circuits each configured to generate a phase up signaland a phase down signal in response to a respective pair of data samplesand intervening transition samples; and a summing circuit configured togenerate an adjustment signal in response to said phase up and phasedown signals of said plurality of phase detector circuits, wherein a sumof said phase up signals and a sum of said phase down signals areweighted to provide a bias to a phase adjustment, wherein (i) said phaseup signals are weighted by said first control parameter, (ii) said phasedown signals are weighted by said second control parameter, and (iii) aratio of said first control parameter and said second control parameteris set based on said target bit error rate of said inner eye.